1. Field of the Invention
The present invention relates to antennas, and more particularly to wideband antennas designed to reduce direct transmit-to-receive coupling effects for use in close range radars.
2. Description of Related Art
Pulse-echo radars are often used to measure range to a target, and recent high-resolution radars have emerged that are capable of centimeter range resolution at short ranges, e.g. on the order of meters. A particular problem with these radars is that they must receive echoes within a matter of nanoseconds after a pulse is transmitted when a target is very close, such as at 1-meter range or less.
If a transmitted pulse directly couples into the radar receiver, it will sum with echo signals to produce a range measurement error. In conventional radar systems, like airport radars, echo signals return many microseconds after direct-coupled pulses have passed and thus can be time gated out. At very short ranges, echoes return very quickly, often while a pulse is still being transmitted or while the antennas are ringing from the transmit pulse. At short ranges, close-in clutter or intercavity coupling effects are very pronounced since they occur right after the large transmit pulse, or main bang. Thus, it is highly desirable to minimize main-bang coupling between transmit and receive antennas in short-range radar applications. Further, strong main-bang pulses coupled into the receiver may create a receiver overload condition, blinding the receiver to nearby echoes.
Ideally, transmit-to-receive antenna coupling should be zero. Unfortunately, coupling between closely mounted antennas can be quite high, typically on the order of -20 dB for two side-by-side horns, and perhaps as great as -6 dB for adjacent dipoles or microstrip patches that are not shielded from each other.
Experiments show that direct antenna-to-antenna coupling must be on the order of -50 dB for radar rangefinders operating with 1-nanosecond wide RF bursts at 5.8 GHz and 1 mm range accuracy. An accuracy of 1 mm is required for tank level radars employed for "custody transfer" measurements--where the cost to fill a petroleum tanker truck from a large storage tank is based on a radar measurement and not a mechanical flowmeter.
U.S. Pat. No. 5,757,320 and U.S. patent application Ser. No. 08/451876 by McEwan describe short range, micropower impulse radars with a swept range gate. The transmit and receive antennas are contained in adjacent shielded cavities to reduce main bang coupling. Conductive or radiative (resistive) damping elements can be added to the cavities, or terminating plates can be attached to the cavity openings.
U.S. Pat. No. 5,754,144 to McEwan describes an ultra-wideband horn antenna with an abrupt radiator which is designed to reduce or eliminate close-in clutter effects. Lips extending from opposed edges of the horn aperture can be used to help launch or receive a clean pulse by controlling trailing pulse ringing due to horn rim effects.
There is essentially no prior art addressing suitable low-coupling antennas since high-accuracy short-range radar ranging is an emerging technology. Digital background subtraction circuits have been added at the output of radar devices to attempt to correct the output signal for coupling effects but are inconvenient to use in many applications. It would be far better to prevent the coupling instead of trying to correct for it.